Stabilized conjugates of uncomplexed subunits of multimeric proteins

ABSTRACT

The present invention provides methods for preparing, and compositions comprising, stabilized protein-polymer conjugates. More particularly, the present invention relates to the stabilization of individual subunits of multisubunit protein complexes by conjugation to polymers. Such conjugation acts to stabilize the individual subunit in its native conformation in liquid medium, which in turn acts to stabilize its biological activity.

This is a continuation-in-part of application Ser. No. 08/730,111, filed Oct. 15, 1996, now abandoned.

TECHNICAL FIELD

The present invention relates to the conjugation of uncomplexed subunits of multisubunit protein complexes with polymers to stabilize their conformation. More specifically, the present invention describes a method of stabilizing these individual subunits via covalent conjugation to a natural or synthetic polymer. The present invention also relates to stabilized conjugates of cardiac troponin I and stabilized conjugates of cardiac troponin T, and methods for their preparation.

BACKGROUND ART

Proteins are composed of long chains of amino acids. The structure of proteins can be considered on four different levels. The primary structure refers to the specific order of amino acids in the polymer chain. The secondary structure refers to the interactions among and between the amino acids in the chain to form such structures as α helices and β pleated sheets. The tertiary structure refers to the three-dimensional structure of the protein, which is also referred to as a protein's conformation. The quaternary structure refers to the spatial arrangement of individual polypeptides or "subunits" of multisubunit proteins.

The native conformation of a protein is only marginally stable. Thus, many proteins which are removed from their native environment and purified undergo conformational changes which can cause a loss of biological activity, such as enzyme activity or antibody binding capacity. In particular, the individual (uncomplexed) subunits of multisubunit protein complexes may undergo dramatic conformational changes when separated from the other subunits of the complex and stored in a liquid medium.

It is often desirable to separate the individual subunits of a multisubunit protein complex, for example to study or exploit the biological activity of each individual subunit. However, this may not be possible if the individual subunits undergo conformational changes in their uncomplexed state that alter their biological activity. Accordingly, it is an object of the present invention to provide a method for stabilizing individual subunits of multisubunit protein complexes.

Troponin is an example of a multisubunit protein complex which consists of three individual subunits; troponin T, troponin C and troponin I. The troponin complex is involved in the calcium-sensitive switch that regulates the interaction of actin and myosin in striated muscles. Troponin T binds the troponin complex to tropomyosin, while troponin I is the inhibitory subunit of the complex. Whereas troponin C from skeletal muscle and cardiac muscle is identical, troponin I and T from these two sources exist as different isoforms, each having a different amino acid sequences and thus a unique structure. Thus, cardiac troponin I (cTnI) and cardiac troponin T (cTnT) are of particular interest as cardiospecific markers.

After myocardial infarction, cTnT levels increase and remain elevated for an extended period. However, it has been reported that in a variety of disease states, cTnT is also expressed in skeletal muscle, which contributes to a lack of cardiospecificity of this protein. Furthermore, uremia, a condition associated with cardiomyopathy, is associated with elevated cTnT. Thus, a lack of absolute cardiospecificity makes this marker less than optimal for use in the early diagnosis of acute myocardial infarction (AMI).

Cardiac TnI is also released after acute myocardial infarction. In contrast to cTnT, cTnI has never been found in a healthy population, which includes marathon runners, in people with skeletal disease, or in patients undergoing non-cardiac operations. Thus, cTnI is a more specific marker for the diagnosis of AMI than other serum proteins.

A variety of immunoassays have been developed that utilize antibodies that can distinguish between the three troponins, and also between their different isoforms. Monoclonal and polyclonal antibodies have been designed and used in immunoassays which can detect the cardiac-specific epitopes formed by the unique amino acid sequence of cTnI. See, for example PCT Patent Application No. WO 96/10076; European Patent No. 394,819 B1; and Adams et al., Circulation 88:101-106 (1993). Larue et al., (Clin. Chem. 39:972-979 (1993)) describe an immunoenzyme assay that is capable of detecting cTnI in the concentration range of 0.2 to 20 μg/L in 30 minutes.

Immunoassays have also been described which are specific for TnT. See, for example, Katus, et al., Circulation, 83(3):902-912 (1991). An immunoassay for TnT is also commercially available from Boehringer Mannheim Corp. Indianapolis, Ind.

Most immunoassays are designed to determine the concentration of a given marker in a patient's serum by comparing immunoassay results with the patient's serum to those obtained with control reagents of known concentration. One limitation in the development of immunoassays for the troponins involves the instability of the troponins in their uncomplexed state. Accordingly, there is a need for stabilized compositions of cTnI and cTnT that can be stored for extended periods of time, while retaining antibody binding capacity for use as control reagents in cTnI and cTnT immunoassays.

Stabilization of proteins via covalent conjugation to various polymers has been described. See, for example, U.S. Pat. Nos. 4,902,502; 5,468,478; 4,806,524; Katre et al., J. Immunol. 144:209-213 (1990); Abuchowski et al., J. Biol. Chem. 252:3582-3586 (1977). The properties conferred on the conjugated protein have been cited as increased in vivo half life, increased stability in solution, increased solubility, decreased susceptibility to proteases and decreased immunogenicity and antigenicity. For example, Nitecki et al. (U.S. Pat. No. 5,089,261) describe conjugating interleukin-2 to polyethylene glycol (PEG) to reduce immunogenicity.

In addition to stabilization of proteins via covalent conjugations to polymers, it has also been demonstrated that synthetic polymers are capable of providing a stabilizing effect via ionic interaction with proteins. For example, Marsh and Danielson (Analyst 120:1091-1096 (1995)) have described that the addition of PEG to an aqueous solutions of the multisubunit enzyme lactate dehydrogenase enhances the ability of the enzyme subunits to remain complexed.

The present invention relates to the finding that covalent conjugation of individual protein subunits of multisubunit proteins to polymers stabilizes the protein subunit, i.e. it maintains its native conformation in a liquid medium for a longer period of time than the equivalent unconjugated individual subunit. In a preferred embodiment, the present invention also relates to cTnI-polymer conjugates and cTnT-polymer conjugates which exhibit stabilized antibody binding capacity.

DISCLOSURE OF THE INVENTION

The present invention concerns stabilization of individual subunits of multisubunit protein complexes. Stabilization is accomplished by conjugating the individual subunits to a polymer. The stabilizing effect of conjugation allows the individual subunit to be stored in liquid medium for longer periods of time than an equivalent unconjugated or "free" individual subunit. This greatly enhances the shelf life of the composition. In a preferred embodiment, the stabilized individual subunits are either cTnl or cTnT, which in their unconjugated form are highly unstable in liquid medium.

Polymers which are useful in the present invention can be naturally occurring or synthetic. Whereas certain synthetic polymers may be preferred for stabilization of free cTnI, as will be discussed below, natural polymers such as serum proteins are preferred for stabilization of cTnT. A particularly preferred class of synthetic polymer is PEG. Other suitable polymers include, but are not limited to polyalkylene glycols, polyoxyethylated polyols, polyvinylpyrrolidone, polyhydroxyethyl methacrylate, polyvinyl alcohols, and polyurethane.

The polymers which are useful in the present invention may vary in molecular weight, and must have a molecular weight which is sufficient to stabilize the individual subunit. This generally requires that the polymer have a molecular weight between 100 and 200,000, more preferably between 1,000 and 40,000, and most preferably between 2,500 and 10,000.

In order to conjugate the polymer to the individual protein subunit, it should be in an "active" form, which means it must contain at least one reactive group capable of reacting with pendant groups on the protein to form a covalent linkage. When the polymer is PEG, a preferred active form is monomethoxy-PEG p-nitrophenyl carbonate.

The ratio of individual subunit to polymer in the conjugation reaction must be sufficient to stabilize the individual subunit. This generally requires that the polymer is provided in a molar concentration which is at least equivalent to the molar concentration of the individual subunit. Preferably, the polymer is provided in excess to ensure that a sufficient number of polymers are covalently attached to the individual subunits.

As an alternative to polymers, monomers (at least some of which are in an active form) can be used to form the protein-polymer conjugates of the present invention, which may polymerize during conjugation and may even attach directly to the protein subunit to afford the desired stability.

Another aspect of the present invention relates to compositions that consist of individual subunit-polymer conjugates in liquid medium. Suitable liquid media include water, aqueous solvents, serum, and mixtures thereof. Preferably, the liquid medium is mammalian serum, and more preferably, it is a mixture of human serum and bovine serum.

Other excipients, such as salts, buffers, proteins, polymers, carbohydrates, preservatives and reducing agents may also be added to the liquid medium.

A preferred embodiment of the present invention relates to stabilized conjugates of cTnI which are useful as control reagent compositions for immunoassays. Preferably, the cTnI conjugates are formed by conjugating a synthetic polymer, such as PEG, to the protein's pendant amine groups. Because cTnI's amine-containing lysine residues are not located in the cardiac-specific N-terminal portion of the protein, conjugation to PEG does not appreciably affect the ability of the cTnI to bind to cardiac-specific anti-cTnI antibodies.

In another embodiment, the present invention relates to a method of stabilizing individual subunits of multisubunit complexes by providing a solution of the individual subunit, adding a multifunctional crosslinking agent to activate the subunit, then simultaneously or subsequently adding a polymer to effect conjugation of the subunit to the polymer via the crosslinking agent. This method of stabilization is particularly preferred for conjugating serum proteins such as albumin to cTnT via glutaraldehyde, although it is also useful for other combinations of polymers and subunits as described herein, as well as with other multifunctional crosslinking agents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram which shows the activation of PEG and coupling to cTnI.

FIGS. 2A and 2B are bar graphs which show the stability of free cTnI and cTnI-PEG conjugates, respectively, at 23° C.

FIG. 3A and 3B are bar graphs which show the stability of free cTnI and cTnI-PEG conjugates, respectively, at 37° C.

FIGS. 4A, 4B and 4C are bar graphs which show the stability of protein conjugated cTnT during storage at 4° C. or 23° C. (4A and 4B, respectively) compared to unconjugated cTnT (4C).

MODES FOR CARRYING OUT THE INVENTION

The present invention relates to methods of stabilizing individual subunits of multisubunit protein complexes via conjugation to polymers. The term "stabilized" as used herein means that the conformation of the protein's subunit when suspended in a liquid medium is maintained for a longer period of time than the equivalent unconjugated or "free" subunit. Stabilization can be measured as a function of the subunit's ability to retain at least one biological activity when suspended in a liquid medium and stored for a period of time, relative to the subunit in its native state. For example, a subunit-polymer conjugate would be considered stabilized if it maintained a level of about 50% of at least one of its biological activities upon storage in a liquid medium, compared to a free subunit. "Biological activity" of a protein subunit can be any biological characteristic attributed at least in part to that particular subunit of a multisubunit complex, such as enzyme activity, ligand binding capacity, substrate recognition, antibody binding, nucleic acid binding and the like.

The hydrophobic interactions between subunits of multisubunit proteins (i.e. a protein's quaternary structure) lend stability to the complex, and also to the conformation of the individual subunits within the complex. Accordingly, isolation of a subunit from a multisubunit protein complex can cause many subunits to undergo conformational changes. For example, individual subunits of multisubunit E. coli enterotoxin have been shown to lose ligand binding capacity when separated from the native complex. Tsuji et al., Microbiol. Immunol. 39:817-819 (1995).

Since a protein's biological activity depends on its conformation, stability of a protein can be measured as a function of the protein's biological activity. For example, specific ligands such as antibodies that recognize the native conformation of a protein can be used to measure changes to that conformation.

Individual subunits of multisubunit proteins may be useful for a variety of purposes, such as for studying or exploiting the biological activities of each. individual subunit in a multisubunit protein complex. For example, in immunoassays, where target specificity can be achieved only by targeting antibodies to a particular subunit of a multisubunit protein complex, the individual subunit is useful as a control reagent for the immunoassay.

Cardiac Troponin I (cTnI)

Troponin is a three-subunit complex of troponin I, T and C. The cardiac isoform of troponin I ("cTnI") is an ideal target for the study and diagnosis of acute myocardial infarction.

In one embodiment, the present invention relates to cTnI-polymer conjugates and methods for the preparation thereof. More particularly, this embodiment of the present invention involves conjugation of cTnI to polymers which act to prevent the uncomplexed cTnI subunit from losing its native conformation and thus losing its binding capacity for cTnI-specific antibodies during storage in a liquid medium.

The nucleic acid sequence of the gene which codes for cTnI and its amino acid sequence have previously been described by Armour et al. (Gene 131:287-292 (1993); GenBank Accession No. M64247, SEQ ID NO:1 and SEQ ID NO:2, respectively).

The term "cTnI" as used herein also intends fragments of cTnI that retain at least one epitope which is recognizable by a monoclonal or polyclonal antibody (or fragment thereof) that preferentially binds to cardiac isoforms of troponin I. Also included in the term "cTnI" are: polypeptides that have amino acid substitutions, deletions or insertions relative to the native or naturally occurring amino acid sequence of cTnI; and fusion proteins that contain all or a fragment of cTnI linked to another protein.

Cardiac Troponin T (cTnT)

Like cTnI, cTnT is also useful as a marker for the study of acute myocardial infarction. Thus, in another embodiment, the present invention relates to cTnT-polymer conjugates and methods for the preparation thereof. Similarly to cTnI, conjugation of cTnT to polymers also acts to maintain cTnT's native conformation.

The nucleic acid sequence of the gene which codes for cTnT and its amino acid sequence have previously been described by Townsend, et al., Genomics, 21(2): 311-316 (1994); GenBank Accession No. X74819; SEQ ID NO.s 3 and 4.

The term "cTnT" as used herein also intends fragments of cTnT that retain at least one epitope which is recognizable by a monoclonal or polyclonal antibody (or fragment thereof) that preferentially binds to cardiac isoforms of troponin T. Also included in the term "cTnT" are: polypeptides that have amino acid substitutions, deletions or insertions relative to the native or naturally occurring amino acid sequence of cTnI; and fusion proteins that contain all or a fragment of cTnT linked to another protein.

Isolating Individual Subunits

Protein subunits such as cTnI and cTnT can be isolated from their natural source in human or animal tissue, or they can be prepared using recombinant techniques. Recombinant techniques are well known in the art and involve isolation and/or synthesis of a polynucleotide encoding all or a fragment of the protein subunit and cloning into a suitable bacterial or eukaryotic expression vector. Such techniques are described in a variety of references, including but not limited to, Molecular Cloning: A Laboratory Manual, 2nd ed., Vol. 1-3, eds. Sambrook et al. Cold Spring Harbor Laboratory Press (1989). After insertion of the vector into a suitable host cell and expression of the desired protein subunit, the protein subunit can be isolated and purified using known techniques. (See, for example, Scopes Protein Purification: Principles and Practice, 2nd Ed., Springer-Verlag, New York. (1987))

In particular, cTnI can be isolated from heart muscle and substantially purified as described in PCT Patent Application No. WO 94/27156. Therein, a procedure is given for affinity purification of cTnI on a troponin C affinity column. Human heart is trimmed of excess fat and valves and cut into 1 cm pieces at 4° C. The resulting tissue is homogenized in one portion with 750 ml extraction buffer (75 mM Tris buffer, pH 8.0, containing 8 M urea, 15 mM mercaptoethanol and 1 mM calcium chloride) at ambient temperature. The homogenate is centrifuged for 30 minutes at 7000×g and the supernatant liquid is filtered through cheesecloth to remove particulate matter. Troponin C coupled to a solid support gel is equilibrated in extraction buffer, and the heart extract is added to the equilibrated troponin C-gel. The resulting suspension is allowed to stir for 80 minutes at ambient temperature and then centrifuged 20 minutes at 7000×g. The pelleted gel is transferred to a column with extraction buffer. The column is washed at ambient temperature with a total of 700 ml of extraction buffer and the purified troponin I is then eluted from the column with elution buffer (75 mM Tris buffer, pH 8.0, containing 8 M urea, 15 mM mercaptoethanol, and 10 mM ethylenediamine tetraacetic acid).

The cTnT subunit can be prepared as generally described above for cTnI, or it can be commercially obtained from a variety of companies such as Research Diagnostics, Inc., Flanders, N.J.; and HyTest, Turku, Finland.

Individual subunits of multisubunit protein complexes other than troponin can also be prepared as described above or by other known methods. In particular, individual subunits can be isolated from their natural source in biological samples by first separating the subunits, then using affinity chromatography with specific antibodies or other subunits of the complex.

Analyzing Individual Subunits

In the present invention, polymer conjugation is used to stabilize the conformation of individual protein subunits. Before proceeding with conjugation, it is preferable to first make sure the protein subunit preparation has the desired conformation. In particular, after isolation of individual subunits from multisubunit complexes, it is desirable to confirm that the isolation procedure did not adversely affect the conformation of the subunit. This can most easily be accomplished by determining whether the individual protein complex still has an acceptable level of the desired biological activity. For example, when the protein subunit is ultimately to be used as a control reagent in an immunoassay, antibody binding capacity should be determined. Testing the binding capacity of a protein for a particular antibody can be accomplished using known techniques. When the individual protein subunit is cTnI/cTnT, cardiac-specific cTnI/cTnT antibodies are used to determine antibody binding capacity. "Cardiac-specific anti-cTnI/cTnT antibodies" refers to antibodies that have a substantially greater binding affinity for cTnI/cTnT than for other troponins or skeletal forms of cTnI/cTnT. Such antibodies to cTnI have been previously described. See, inter alia, Katus et al., European Patent No. EP 394,819; Takahashi et al., PCT Patent Application No. WO 96/10076; Larue et al., Clin. Chem. 39:972-979 (1993); and Bodor et al., Clin. Chem. 38:2203-2214 (1992). Antibodies which are specific for cTnT have also been described. See for example, Bodor, et al., Clinical Chemistry, 43(3):476-484 (1997), which describes a polyclonal goat anti-cTnT antibody (G136-C; Fortron BioScience, Morrisville, N.C.) which was developed against the N terminal amino acids 3-15 of human cTnT and which reportedly reacts <0.4% with skeletal isoforms. The N-terminal amino acid sequence is shown as follows:

    Ile-Glu-Glu-Val-Val-Glu-Glu-Tyr-Glu-Glu-Glu-Glu-Gln        (SEQ ID NO: 5)

In addition, Muller-Bardorff, et al. (Clinical Chemistry, 43(3)458-466 (1997)) describe a monoclonal antibody pair (M7 and M11.7)which can be used in a sandwich assay for cTnT which reportedly cross reacts <0.3% with skeletal isoforms.

Polymers

Stabilization of individual protein subunits is achieved by conjugating the protein subunit to a polymer to prevent conformational changes of the protein subunit in the uncomplexed state and thus loss of its biological activity. The polymers of the present invention must have a high enough molecular weight to effectively stabilize the conformation of the protein subunit. Preferably, the molecular weight is between 100 to 200,000, more preferably between 1,000 and 40,000, and most preferably between 2,500 and 10,000.

The polymer may be already formed prior to conjugation to the protein subunit (i.e. the monomeric units comprising the polymer may already be covalently attached), or in the alternative, the polymer may be formed during conjugation by using monomeric units such as monosaccharides, amino acids, or alkyl groups (C2 to C20 substituted or unsubstituted, branched or unbranched, saturated, partially saturated or unsaturated) which are "polymerized" (covalently attached to one another) during conjugation to the protein, or directly attached to the protein in their monomeric form. In any event, the term "polymer" and "polymer-protein conjugate" as used herein is intended to cover any of the aforementioned variations.

The polymer can be naturally occurring or synthetic. Examples of naturally occurring polymers include proteins, glycopeptides, polysaccharides such as dextran and lipids. In the case of cTnI, the polymer is preferably a synthetic polymer. Examples of synthetic polymers which are suitable for use in the present invention include, but are not limited to, polyalkyl glycols (PAG) such as PEG, polyoxyethylated polyols (POP) such as polyoxyethylated glycerol (POG), polytrimethylene glycol (PTG), polypropylene glycol (PPG), polyhydroxyethyl methacrylate, polyvinyl alcohol (PVA), polyacrylic acid. polyethyloxazoline, polyacrylamide, polyvinylpyrrolidone (PVP), polyamino acids, polyurethane and polyphosphazene. The synthetic polymers can also be linear or branched, substituted or unsubstituted, homopolymeric, or co-polymers of two or more different synthetic monomers.

The synthetic polymers of the present invention have the following generic structure:

    R.sub.1 --(X--R.sub.2).sub.a --R.sub.3

where R₁ and R₃ are the same or different and are H, H₃ C, OH, R₂ or a reactive group (as described below); where R₂ is a linear or branched substituted or unsubstituted alkyl group; where X is O (in which case the synthetic polymer may be a polyoxyalkylene) or X is NH(C═O) (in which case the synthetic polymer may be a polyamine), or X is absent (in which case the synthetic polymer may be a polyalkylene); and a is an integer between 1 and 1,000.

A preferred class of synthetic polymers are the polyethylene glycols given by the formula:

    R.sub.1 O--(CH.sub.2 CH.sub.2 O).sub.a --R.sub.3

where R₁, R₃ and a are as described above. The term "polyethylene glycol" (PEG) includes both unsubstituted (R₁ =H and R₃ =OH) as well as substituted polyethylene glycol.

Although hydrophilic polymers are preferred, it is also possible to use hydrophobic polymers, such as activated suberate or proprionate derivatives, or mixtures of hydrophilic or hydrophobic polymers.

Naturally occurring proteins which are preferred for conjugation to cTnT are serum proteins, such as albumin. For convenience, the naturally occurring protein used for conjugation to cTnT can actually be a heterogeneous mixture of proteins, such as one which would be found in animal sera (for example, bovine or human), or in a synthetically produced serum substitute.

Polymer Activation

In order to conjugate the protein subunit to a polymer (which is either already polymerized or in its monomeric form at the time of conjugation), the polymer may first need to be activated. "Activated" means the preliminary attachment of a reactive group onto the polymer. A polymer which has been activated, as well as a polymer which already contains at least one reactive group is referred to as being "active". Many synthetic polymers do not normally contain reactive groups which will react with a protein's pendant groups. For example, unsubstituted PEG has a hydroxyl group at each end of the linear polymer chain, one or both of which must first be activated before conjugation to a protein. To prevent the potential for cross-linking, it is desirable to activate only one of PEG's two hydroxyl termini to form a "monofunctional" substituted PEG. This can be accomplished by blocking one of the two hydroxyl groups with a methoxy group to form monomethoxypolyethylene glycol (mPEG).

Techniques for activating polymers prior to protein conjugation are known to those skilled in the art. See, for example, Greg T. Hermanson, Bioconjugate Techniques, p. 605-619 (1996) Academic Press. For example, activation of hydroxyl groups in PEG or mPEG as well as in other natural or synthetic polymers, can be accomplished using trichloro-s-triazine (TsT; cyanuric acid). (See, inter alia, Abuchowski et al., J. Biol. Chem. 252:3582-3586 (1977) and Abuchowski et al., J. Biol. Chem. 252:3578-3581 (1977)). Another method of activating hydroxyl groups is through formation of an amine reactive N-hydroxyl succinimidyl-(NHS) or p-nitrophenyl (Np) carbonate active ester. (See Zalipsky et al., Biotechnol. Appl. Biochem. 15:100-114 (1992).)

Similar activation can be achieved when the hydroxyl-containing polymer is first reacted with a cyclic anhydride (succinic or glutaric anhydride) and then the formed carboxyl modified product is coupled with N-hydroxyl succinimide in the presence of carbodiimides resulting in succinimidyl succinate or glutarate type active esters. (See Abuchowski et al., Cancer Biochem. Biophys. 7:175-186 (1984).)

A further method for activating a polymer's hydroxyl groups is through formation of an imidazolyl carbamate intermediate by reaction with N,N'-carbonyldiimidazole (CDI). The CDI-activated polymer reacts with amine groups of protein to form a stable N-alkyl carbamate linkage identical to that formed with succinimidyl carbonate chemistry described above. (See Beauchamp et al., Anal. Biochem. 131:25-33 (1983).)

For additional methods on activating synthetic polymers, such as PEG, see inter alia, U.S. Pat. Nos. 5,349,001; 5,359,030; and 5,446,090.

When the polymer to be conjugated to the protein subunit is a natural polymer such as a serum protein, it is generally preferred to activate the protein subunit by attachment of a multifunctional crosslinking agent as is more fully described in the following section.

Polymer-protein Conjugation

After activation of the polymer (if necessary), the polymer is conjugated to the individual protein subunit. In general, polymers can be covalently attached to proteins via pendant groups in the protein chain, such as primary amino groups. carboxyl groups, aromatic rings, or thiol groups, all of which may already be present, or can be added by preliminary chemical modification of the protein or by modifying a protein's amino acid sequence, using known molecular biology methods. One of the most frequently used and convenient methods involves attachment of the polymer to the protein's free amino groups in lysine residues (ε-amino group) or to the protein's N-terminal amino acid (α-amino group). This approach is preferred when the protein's amino groups are located in segments of the protein which are not crucial to maintaining the desired biological activity, such as is the case with cTnI. The discussion which follows outlines two representative embodiments of the present invention. It is to be noted that the methodologies presented for cTnI are equally applicable for cTnT, and vice versa, as well as other protein subunits.

The ratio of polymer to protein to be used to carry out the conjugation depends on the characteristics (structure, size, charge, reactivity) of the polymer, as well as the characteristics (number and location of pendant groups, nature of the biological activity) of the individual subunit. It would be a matter of routine experimentation to determine the appropriate ratio by varying the ratio to optimize biological activity and conjugate stability.

A. cTnI-polymer Conjugates

There are three isoforms of troponin I: slow skeletal, fast skeletal and cardiac troponin I (herein referred to as "cTnI"). The three isoformns of troponin I in rabbits show about 40% sequence difference at the amino acid level, as described by Wilkinson and Grand Nature, 271:31-35 (1978). The nuclectide and amino acid sequences of human cTnI are given as SEQ ID NO:1 and SEQ ID NO:2, respectively. cTnI also contains a cardiac-specific 31 amino acid N-terminal sequence, which is completely missing from either of the two skeletal isoforms, as shown below:

    Met-Ala-Asp-Gly-Ser-Ser-Asp-Ala-Ala-Arg-Glu-Pro-Arg-Pro-Ala-Pro -Ala-Pro-Ile-Arg-Arg-Arg-Ser-Ser-Asn-Tyr-Arg-Ala-Tyr-Ala-Thr-(SEQ ID NO: 6)

Since this cardiac-specific segment contains no lysine residues, it is possible to conjugate polymers to amino groups located elsewhere on the protein without appreciably affecting the ability of the cTnI-PEG conjugate to bind to cardiac-specific anti-cTnI antibodies.

In the case of cTnI and PEG, the preferred ratio is between 1 and 20 moles of active PEG per mole of cTnI, more preferably between 6 and 16, and most preferably between 10 and 14. The resultant cTnI-PEG conjugate is considered stable if it retains at least 85% of its biological activity when stored at 4° C. for a period of about 30 days. In the case of cTnI, "biological activity" refers to the ability of cTnI to be recognized by cTnI-specific antibodies. The cTnI-PEG conjugate is preferentially stored at -20° C. to 23° C., more preferably between about 2° C. to about 8° C. The cTnI-PEG conjugate may also be lyophilized or stored under an inert gas such as argon, nitrogen, etc.

B. cTnT-polymer Conjugates

In addition to conjugations which involve preactivation of the polymer, it is also possible to effect conjugation via a crosslinking agent. The crosslinking agent is preferably multifunctionally active and more preferably difunctionally active, which means that it contains more than one ("multi-") or two ("di-") reactive groups. The crosslinking agent can be one of the synthetic polymers described above which has been activated to contain two or more reactive groups, which can then be used as a linker to conjugate the protein subunit to another synthetic or natural polymer.

The crosslinking agent may also be a more conventional chemical crosslinking agent, including but not limited to, glutaraldehyde, formaldehyde, divinyl sulfane, carbodiimides, epoxides and imidazole. In the case of cTnT, glutaraldehyde is preferred.

Protein-polymer Conjugate Compositions

Protein-polymer compositions can be formulated by diluting, dissolving or concentrating and rediluting the protein-polymer conjugate in a suitable liquid medium, such as water, aqueous solvents, serum, or mixtures thereof. Excipients can also be added to the liquid medium to further optimize the formulation. For example, cyclodextrin or other carbohydrates can be added to the protein-polymer conjugate to inhibit the rate of intermolecular aggregation during storage in the liquid medium.

Other optional components include, but are not limited to, casein, albumin, gelatin or other proteins. Buffers can be added to stabilize the pH of the liquid medium. Protease inhibitors such as phenyl methyl sulfonyl fluoride, leupeptin, pepstatin may also be included. Preservatives such as Amphotericin B, cycloheximide, chloramphenicol, bacitracin, gentamycin, chloroheximide, sodium azide, trimethoprim, sulphomethaxozole may also be included. Additionally, reducing agents such as glutathione may be added.

It may also be desirable to sterilize the protein-polymer conjugate composition after formulation, such as by means of filtration. Filtration may be accomplished by passing the protein-polymer conjugate composition through a suitable size control device, such as a filter, molecular sieves, resins, hollow fibers, and spiral cartridge exclusions. Preferably, a 0.2 micron aseptic filter is used.

Preferably, the liquid medium is mammalian serum, more preferably a mixture of bovine and human serum, which has been filter sterilized, and which also contains carbohydrates, and antimicrobial agents. For a description of such a "modified" human serum, see U.S. Pat. No. 5,556,788. The sera used in the examples which follow were modified.

EXAMPLES Example I

Conjugation of cTnI to PEG

cTnI from Scripps Laboratories (La Jolla, Calif.) was dissolved in water at 4° C. to a concentration of 1 mg/ml. To 50 μl of this cTnI solution, an equal volume of conjugation buffer (0.1 M sodium phosphate, pH=7.5) was added, and the resultant mixture was kept on ice. Meanwhile, the activated polymer solution was prepared by dissolving 5 mg methoxypolyethylene glycol p-nitrophenyl carbonate (mPEG-ONp, avg. MW=5,000) in 1 ml of ice-cold conjugation buffer. 42 μl of activated polymer solution (200 fold molar excess) were added to the buffered cTnI solution, and the mixture was incubated for 18 hours at 4° C. with slow end-to-end mixing. The cTnI-PEG conjugate thus formed was frozen and stored at -20° C. until use.

Example II

Stability of cTnI-PEG Conjugates

The stability of the cTnI-PEG conjugates prepared according to Example I was compared to that of unconjugated cTnI. Both the conjugated and unconjugated cTnI preparations were diluted in a liquid medium of 50% human and 50% bovine serum, and stored at various temperatures. At the indicated times, samples were removed and analyzed as described in Example III. The results are shown in FIGS. 2 and 3 as a percentage of the concentration of cTnI at a given time compared to the initial concentration (100%). The results depicted in FIG. 2 indicate that, while unconjugated cTnI stored at 23° C. for 20 days retains only about 20% of its initial antibody binding capacity, the cTnI-PEG conjugate retains essentially 100% activity when stored under the same conditions. Similar results are obtained when the unconjugated cTnI ant the cTnI-PEG conjugates are stored at 37° C. as depicted in FIG. 3. Storage at 4° C. of unconjugated cTnI typically results in the loss of at least 25% of its original antibody binding activity, while the cTnI-PEG conjugate retains 100% of its activity.

Example III

Measuring Antibody Binding Capacity

Antibody binding capacity is measured using either of the two commercially available assays for the quantitative determination of cTnI levels in serum and plasma according to the manufacturer's specifications. These assays are: the Stratus® cardiac Troponin-I fluorometric enzyme immunoassay (Baxter Diagnostics Inc./Dade International Inc., Miami, Fla.); and the OPUS® Troponin I assay (Behring Diagnostics Inc., Westwood, Mass.).

Example IV

Conjugation of cTnT to Serum Proteins

Glutarldehyde was added to a 400 mg/mL solution of cTnT from human heart tissue (HyTest, Turku, Finland) to a concentration of 0.1%. After mixing well and refrigerating, the glutarldehyde activated cTnT was diluted 1:10 in either human serum or bovine serum. It is hypothesized that albumin, being the most prevalent protein which is present in the serum, is conjugated to the cTnT via the glutaraldehyde.

Example V

Stability of cTnT-serum Protein Conjugates

Stability studies were performed as described above for cTnI, and the results are shown in FIGS. 4A-C. As depicted, the serum conjugated cTnT is stable for more then 6 months at either 4° C. or 23° C.

Modifications of the above-described modes for carrying out the invention that are obvious to persons of skill in the field of protein chemistry are intended to be within the scope of the following claims. All publications, patents, and patent applications cited in this specification are incorporated herein by reference as if each such publication, patent or patent application were specifically and individually indicated to be incorporated herein by reference.

    __________________________________________________________________________     #             SEQUENCE LISTING                                                 - (1) GENERAL INFORMATION:                                                     -    (iii) NUMBER OF SEQUENCES: 6                                              - (2) INFORMATION FOR SEQ ID NO:1:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 633 base                                                           (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ix) FEATURE:                                                                      (A) NAME/KEY: Coding Se - #quence                                              (B) LOCATION: 1...630                                                          (D) OTHER INFORMATION:                                               -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                  - ATG GCG GAT GGG AGC AGC GAT GCG GCT AGG GA - #A CCT CGC CCT GCA CCA            48                                                                           Met Ala Asp Gly Ser Ser Asp Ala Ala Arg Gl - #u Pro Arg Pro Ala Pro            #                15                                                            - GCC CCA ATC AGA CGC CGC TCC TCC AAC TAC CG - #C GCT TAT GCC ACG GAG            96                                                                           Ala Pro Ile Arg Arg Arg Ser Ser Asn Tyr Ar - #g Ala Tyr Ala Thr Glu            #            30                                                                - CCG CAC GCC AAG AAA AAA TCT AAG ATC TCC GC - #C TCG AGA AAA TTG CAG           144                                                                           Pro His Ala Lys Lys Lys Ser Lys Ile Ser Al - #a Ser Arg Lys Leu Gln            #        45                                                                    - CTG AAG ACT CTG CTG CTG CAG ATT GCA AAG CA - #A GAG CTG GAG CGA GAG           192                                                                           Leu Lys Thr Leu Leu Leu Gln Ile Ala Lys Gl - #n Glu Leu Glu Arg Glu            #    60                                                                        - GCG GAG GAG CGG CGC GGA GAG AAG GGG CGC GC - #T CTG AGC ACC CGC TGC           240                                                                           Ala Glu Glu Arg Arg Gly Glu Lys Gly Arg Al - #a Leu Ser Thr Arg Cys            #80                                                                            - CAG CCG CTG GAG TTG GCC GGG CTG GGC TTC GC - #G GAG CTG CAG GAC TTG           288                                                                           Gln Pro Leu Glu Leu Ala Gly Leu Gly Phe Al - #a Glu Leu Gln Asp Leu            #                95                                                            - TGC CGA CAG CTC CAC GCC CGT GTG GAC AAG GT - #G GAT GAA GAG AGA TAC           336                                                                           Cys Arg Gln Leu His Ala Arg Val Asp Lys Va - #l Asp Glu Glu Arg Tyr            #           110                                                                - GAC ATA GAG GCA AAA GTC ACC AAG AAC ATC AC - #G GAG ATT GCA GAT CTG           384                                                                           Asp Ile Glu Ala Lys Val Thr Lys Asn Ile Th - #r Glu Ile Ala Asp Leu            #       125                                                                    - ACT CAG AAG ATC TTT GAC CTT CGA GGC AAG TT - #T AAG CGG CCC ACC CTG           432                                                                           Thr Gln Lys Ile Phe Asp Leu Arg Gly Lys Ph - #e Lys Arg Pro Thr Leu            #   140                                                                        - CGG AGA GTG AGG ATC TCT GCA GAT GCC ATG AT - #G CAG GCG CTG CTG GGG           480                                                                           Arg Arg Val Arg Ile Ser Ala Asp Ala Met Me - #t Gln Ala Leu Leu Gly            145                 1 - #50                 1 - #55                 1 -        #60                                                                            - GCC CGG GCT AAG GAG TCC CTG GAC CTG CGG GC - #C CAC CTC AAG CAG GTG           528                                                                           Ala Arg Ala Lys Glu Ser Leu Asp Leu Arg Al - #a His Leu Lys Gln Val            #               175                                                            - AAG AAG GAG GAC ACC GAG AAG GAA AAC CGG GA - #G GTG GGA GAC TGG CGC           576                                                                           Lys Lys Glu Asp Thr Glu Lys Glu Asn Arg Gl - #u Val Gly Asp Trp Arg            #           190                                                                - AAG AAC ATC GAT GCA CTG AGT GGA ATG GAG GG - #C CGC AAG AAA AAG TTT           624                                                                           Lys Asn Ile Asp Ala Leu Ser Gly Met Glu Gl - #y Arg Lys Lys Lys Phe            #       205                                                                    #        633                                                                   Glu Ser                                                                            210                                                                        - (2) INFORMATION FOR SEQ ID NO:2:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #acids    (A) LENGTH: 210 amino                                                          (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: protein                                              -      (v) FRAGMENT TYPE: internal                                             -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                  - Met Ala Asp Gly Ser Ser Asp Ala Ala Arg Gl - #u Pro Arg Pro Ala Pro          #                15                                                            - Ala Pro Ile Arg Arg Arg Ser Ser Asn Tyr Ar - #g Ala Tyr Ala Thr Glu          #            30                                                                - Pro His Ala Lys Lys Lys Ser Lys Ile Ser Al - #a Ser Arg Lys Leu Gln          #        45                                                                    - Leu Lys Thr Leu Leu Leu Gln Ile Ala Lys Gl - #n Glu Leu Glu Arg Glu          #    60                                                                        - Ala Glu Glu Arg Arg Gly Glu Lys Gly Arg Al - #a Leu Ser Thr Arg Cys          #80                                                                            - Gln Pro Leu Glu Leu Ala Gly Leu Gly Phe Al - #a Glu Leu Gln Asp Leu          #                95                                                            - Cys Arg Gln Leu His Ala Arg Val Asp Lys Va - #l Asp Glu Glu Arg Tyr          #           110                                                                - Asp Ile Glu Ala Lys Val Thr Lys Asn Ile Th - #r Glu Ile Ala Asp Leu          #       125                                                                    - Thr Gln Lys Ile Phe Asp Leu Arg Gly Lys Ph - #e Lys Arg Pro Thr Leu          #   140                                                                        - Arg Arg Val Arg Ile Ser Ala Asp Ala Met Me - #t Gln Ala Leu Leu Gly          145                 1 - #50                 1 - #55                 1 -        #60                                                                            - Ala Arg Ala Lys Glu Ser Leu Asp Leu Arg Al - #a His Leu Lys Gln Val          #               175                                                            - Lys Lys Glu Asp Thr Glu Lys Glu Asn Arg Gl - #u Val Gly Asp Trp Arg          #           190                                                                - Lys Asn Ile Asp Ala Leu Ser Gly Met Glu Gl - #y Arg Lys Lys Lys Phe          #       205                                                                    - Glu Ser                                                                          210                                                                        - (2) INFORMATION FOR SEQ ID NO:3:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #pairs    (A) LENGTH: 867 base                                                           (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: double                                                       (D) TOPOLOGY: linear                                                 -     (ix) FEATURE:                                                                      (A) NAME/KEY: Coding Se - #quence                                              (B) LOCATION: 1...864                                                          (D) OTHER INFORMATION:                                               -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                  - ATG TCT GAC ATA GAA GAG GTG GTG GAA GAG TA - #C GAG GAG GAG GAG CAG            48                                                                           Met Ser Asp Ile Glu Glu Val Val Glu Glu Ty - #r Glu Glu Glu Glu Gln            #                15                                                            - GAA GAA GCA GCT GTT GAA GAG CAG GAG GAG GC - #A GCG GAA GAG GAT GCT            96                                                                           Glu Glu Ala Ala Val Glu Glu Gln Glu Glu Al - #a Ala Glu Glu Asp Ala            #            30                                                                - GAA GCA GAG GCT GAG ACC GAG GAG ACC AGG GC - #A GAA GAA GAT GAA GAA           144                                                                           Glu Ala Glu Ala Glu Thr Glu Glu Thr Arg Al - #a Glu Glu Asp Glu Glu            #        45                                                                    - GAA GAG GAA GCA AAG GAG GCT GAA GAT GGC CC - #A ATG GAG GAG TCC AAA           192                                                                           Glu Glu Glu Ala Lys Glu Ala Glu Asp Gly Pr - #o Met Glu Glu Ser Lys            #    60                                                                        - CCA AAG CCC AGG TCG TTC ATG CCC AAC TTG GT - #G CCT CCC AAG ATC CCC           240                                                                           Pro Lys Pro Arg Ser Phe Met Pro Asn Leu Va - #l Pro Pro Lys Ile Pro            #80                                                                            - GAT GGA GAG AGA GTG GAC TTT GAT GAC ATC CA - #C CGG AAG CGC ATG GAG           288                                                                           Asp Gly Glu Arg Val Asp Phe Asp Asp Ile Hi - #s Arg Lys Arg Met Glu            #                95                                                            - AAG GAC CTG AAT GAG TTG CAG GCG CTG ATT GA - #G GCT CAC TTT GAG AAC           336                                                                           Lys Asp Leu Asn Glu Leu Gln Ala Leu Ile Gl - #u Ala His Phe Glu Asn            #           110                                                                - AGG AAG AAA GAG GAG GAG GAG CTC GTT TCT CT - #C AAA GAC AGG ATC GAG           384                                                                           Arg Lys Lys Glu Glu Glu Glu Leu Val Ser Le - #u Lys Asp Arg Ile Glu            #       125                                                                    - AGA CGT CGG GCA GAG CGG GCC GAG CAG CAG CG - #C ATC CGG AAT GAG CGG           432                                                                           Arg Arg Arg Ala Glu Arg Ala Glu Gln Gln Ar - #g Ile Arg Asn Glu Arg            #   140                                                                        - GAG AAG GAG CGG CAG AAC CGC CTG GCT GAA GA - #G AGG GCT CGA CGA GAG           480                                                                           Glu Lys Glu Arg Gln Asn Arg Leu Ala Glu Gl - #u Arg Ala Arg Arg Glu            145                 1 - #50                 1 - #55                 1 -        #60                                                                            - GAG GAG GAG AAC AGG AGG AAG GCT GAG GAT GA - #G GCC CGG AAG AAG AAG           528                                                                           Glu Glu Glu Asn Arg Arg Lys Ala Glu Asp Gl - #u Ala Arg Lys Lys Lys            #               175                                                            - GCT TTG TCC AAC ATG ATG CAT TTT GGG GGT TA - #C ATC CAG AAG CAG GCC           576                                                                           Ala Leu Ser Asn Met Met His Phe Gly Gly Ty - #r Ile Gln Lys Gln Ala            #           190                                                                - CAG ACA GAG CGG AAA AGT GGG AAG AGG CAG AC - #T GAG CGG GAA AAG AAG           624                                                                           Gln Thr Glu Arg Lys Ser Gly Lys Arg Gln Th - #r Glu Arg Glu Lys Lys            #       205                                                                    - AAG AAG ATT CTG GCT GAG AGG AGG AAG GTG CT - #G GCC ATT GAC CAC CTG           672                                                                           Lys Lys Ile Leu Ala Glu Arg Arg Lys Val Le - #u Ala Ile Asp His Leu            #   220                                                                        - AAT GAA GAT CAG CTG AGG GAG AAG GCC AAG GA - #G CTG TGG CAG AGC ATC           720                                                                           Asn Glu Asp Gln Leu Arg Glu Lys Ala Lys Gl - #u Leu Trp Gln Ser Ile            225                 2 - #30                 2 - #35                 2 -        #40                                                                            - TAT AAC TTG GAG GCA GAG AAG TTC GAC CTG CA - #G GAG AAG TTC AAG CAG           768                                                                           Tyr Asn Leu Glu Ala Glu Lys Phe Asp Leu Gl - #n Glu Lys Phe Lys Gln            #               255                                                            - CAG AAA TAT GAG ATC AAT GTT CTC CGA AAC AG - #G ATC AAC GAT AAC CAG           816                                                                           Gln Lys Tyr Glu Ile Asn Val Leu Arg Asn Ar - #g Ile Asn Asp Asn Gln            #           270                                                                - AAA GTC TCC AAG ACC CGC GGG AAG GCT AAA GT - #C ACC GGG CGC TGG AAA T         865                                                                           Lys Val Ser Lys Thr Arg Gly Lys Ala Lys Va - #l Thr Gly Arg Trp Lys            #       285                                                                    #             867                                                              - (2) INFORMATION FOR SEQ ID NO:4:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #acids    (A) LENGTH: 288 amino                                                          (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (ii) MOLECULE TYPE: protein                                              -      (v) FRAGMENT TYPE: internal                                             -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                  - Met Ser Asp Ile Glu Glu Val Val Glu Glu Ty - #r Glu Glu Glu Glu Gln          #                15                                                            - Glu Glu Ala Ala Val Glu Glu Gln Glu Glu Al - #a Ala Glu Glu Asp Ala          #            30                                                                - Glu Ala Glu Ala Glu Thr Glu Glu Thr Arg Al - #a Glu Glu Asp Glu Glu          #        45                                                                    - Glu Glu Glu Ala Lys Glu Ala Glu Asp Gly Pr - #o Met Glu Glu Ser Lys          #    60                                                                        - Pro Lys Pro Arg Ser Phe Met Pro Asn Leu Va - #l Pro Pro Lys Ile Pro          #80                                                                            - Asp Gly Glu Arg Val Asp Phe Asp Asp Ile Hi - #s Arg Lys Arg Met Glu          #                95                                                            - Lys Asp Leu Asn Glu Leu Gln Ala Leu Ile Gl - #u Ala His Phe Glu Asn          #           110                                                                - Arg Lys Lys Glu Glu Glu Glu Leu Val Ser Le - #u Lys Asp Arg Ile Glu          #       125                                                                    - Arg Arg Arg Ala Glu Arg Ala Glu Gln Gln Ar - #g Ile Arg Asn Glu Arg          #   140                                                                        - Glu Lys Glu Arg Gln Asn Arg Leu Ala Glu Gl - #u Arg Ala Arg Arg Glu          145                 1 - #50                 1 - #55                 1 -        #60                                                                            - Glu Glu Glu Asn Arg Arg Lys Ala Glu Asp Gl - #u Ala Arg Lys Lys Lys          #               175                                                            - Ala Leu Ser Asn Met Met His Phe Gly Gly Ty - #r Ile Gln Lys Gln Ala          #           190                                                                - Gln Thr Glu Arg Lys Ser Gly Lys Arg Gln Th - #r Glu Arg Glu Lys Lys          #       205                                                                    - Lys Lys Ile Leu Ala Glu Arg Arg Lys Val Le - #u Ala Ile Asp His Leu          #   220                                                                        - Asn Glu Asp Gln Leu Arg Glu Lys Ala Lys Gl - #u Leu Trp Gln Ser Ile          225                 2 - #30                 2 - #35                 2 -        #40                                                                            - Tyr Asn Leu Glu Ala Glu Lys Phe Asp Leu Gl - #n Glu Lys Phe Lys Gln          #               255                                                            - Gln Lys Tyr Glu Ile Asn Val Leu Arg Asn Ar - #g Ile Asn Asp Asn Gln          #           270                                                                - Lys Val Ser Lys Thr Arg Gly Lys Ala Lys Va - #l Thr Gly Arg Trp Lys          #       285                                                                    - (2) INFORMATION FOR SEQ ID NO:5:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #acids    (A) LENGTH: 13 amino                                                           (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                  - Ile Glu Glu Val Val Glu Glu Tyr Glu Glu Gl - #u Glu Gln                      #                10                                                            - (2) INFORMATION FOR SEQ ID NO:6:                                             -      (i) SEQUENCE CHARACTERISTICS:                                           #acids    (A) LENGTH: 31 amino                                                           (B) TYPE: amino acid                                                           (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                 -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                  - Met Ala Asp Gly Ser Ser Asp Ala Ala Arg Gl - #u Pro Arg Pro Ala Pro          #                15                                                            - Ala Pro Ile Arg Arg Arg Ser Ser Asn Tyr Ar - #g Ala Tyr Ala Thr              #            30                                                                __________________________________________________________________________ 

We claim:
 1. A method of stabilizing an individual subunit of a multisubunit protein complex in liquid medium wherein the individual subunit is cardiac troponin I (cTnI), the method comprising the steps of:a) providing a solution of cTnI; and b) mixing said solution of cTnI with an active polymer for a time sufficient and under conditions suitable to form a stabilized cTnI-polymer conjugate.
 2. The method according to claim 1, wherein the polymer is a synthetic polymer.
 3. The method according to claim 2, wherein the synthetic polymer is polythene glycol (PEG).
 4. The method according to claim 2, wherein the synthetic polymer is selected from the group consisting of polyalkylene glycols, polyoxyethylated polyols, polyvinylpyrrolidone, polyhydroxyethyl methacrylate, polyvinyl alcohols, and polyurethane.
 5. The method according to claim 1, wherein the polymer has an average molecular weight between 100 and 200,000 kilodaltons.
 6. The method according to claim 1, wherein the polymer has an average molecular weight between 1,000 and 40,000 kilodaltons.
 7. The method according to claim 1, wherein the polymer has an average molecular weight between 2,500 and 10,000 kilodaltons.
 8. The method according to claim 3, wherein the PEG has an average molecular weight between 2,500 and 10,000 kiodaltons.
 9. The method according to claim 3, wherein the PEG has an average molecular weight of about 5,000 kilodaltons.
 10. The method according to claim 3, wherein the PEG is conjugated with the cTnI in a molar ratio of about 1:1 to 20:1.
 11. The method according to claim 3, wherein the PEG is monomethoxy-PEG p-nitrophenyl carbonate.
 12. The method according to claim 1, wherein the cTnI is a fragment of cTnI, and wherein said fragment is capable of being recognized by an antibody specific for cTnI.
 13. The method of claim 1, wherein the polymer is in its monomeric form when mixed with said individual subunit.
 14. A method of stabilizing an individual subunit of a multisubunit protein complex in liquid medium wherein the individual subunit is cardiac troponin I (cTnI), the method comprising the steps of:a) providing a solution of cTnI; b) adding a multifunctional crosslinking agent to said solution to form an active cTnI; and c) mixing said active cTnI with a polymer for a time sufficient and under conditions suitable to form a stabilized cTnI-polymer conjugate.
 15. The method of claim 14, wherein the multifunctional crosslinking agent is glutaraldehyde.
 16. The method of claim 14, wherein the multifunctional crosslinking agent is a polyethylene glycol. 